WASHINGTON – Senate Republicans are racing toward crucial votes over their tax plan but still have numerous major decisions to make before they secure the 50 votes they need to pass the bill.

The Senate Budget Committee plans to hold a procedural vote on the tax measure on Tuesday, and if that falters the entire process will stall. If the panel approves the measure, then the broader tax package could come to a vote Thursday or Friday. But Republicans have a lot of work to do on designing specific changes before that point.

The biggest new headache for the White House and GOP leaders is a resolute defiance from a group of conservative senators who are worried about the tax bill’s potential to explode the debt.

Sen. James Lankford, R-Okla., told reporters Monday he is working on inserting a provision into the bill that would effectively create a revenue backstop in case the economic growth that Republicans have promised will occur never materializes.

The Senate tax cut bill would add between $1.4 trillion and $1.5 trillion to the debt over 10 years, but there are a number of expiring tax cuts that many Republicans have said would likely be extended, and those changes would add hundreds of billions of additional dollars to the debt. The U.S. government currently has more than $20 trillion in outstanding debt, and Lankford said he is concerned about adding even more.

Lankford is working on the changes with Sen. Bob Corker, R-Tenn., and Sen. Jerry Moran, R-Kansas. Kansas Republicans slashed taxes so severely in 2012 that they had to cut a number of state programs, and they eventually reversed themselves earlier this year to bring revenue back.

“What if this doesn’t work?” Lankford asked hypothetically Monday referring to tax cuts in the Senate plan. “What changes might be needed in the tax code in the days ahead to be able to adjust in what scenario. So if the revenue’s not coming in, should the rates change? All of those are in conversation.”

He wouldn’t explain precisely what these new rates might look like or how they would be triggered. Lankford’s comments suggest that one solution he is eyeing would be automatically raising tax rates if tax revenue come in below the GOP’s optimistic forecasts.

“Because we’re going through a lot of different options on that I’d rather not try to get into the details,” he said. “But there are a lot of ways to be able to do a backstop and we’re trying to be able to work through what’s the best way that’s predictable for everyone of how this would work but it’s also making sure that we actually have a backstop in place and not just hope that this works out.”

Lankford is considered an unwavering conservative, and his lingering concerns with the tax bill show how many issues the White House and GOP leaders are trying to appease. Republicans are trying to rush the tax bill through Congress with little debate or examination of how the bevy of changes would work, a process that lends itself to many members having fundamental questions about what the impact would be.

The fact that Lankford is working with Corker and Moran gives him even more sway, as the bloc of three has enough sway to single-handedly kill the bill. Republicans control 52 votes in the 100-seat Senate. They are trying to pass the bill with a simple majority of votes, and they can use Vice President Mike Pence as a tiebreaker. But if they lose three Republican members the bill will fall, unless a Democrat breaks ranks to support the bill, which appears unlikely at this time.

Lankford, Corker, and Moran aren’t the only ones negotiating changes.

The White House is frantically trying to stop Sens. Ron Johnson, R-Wis., and Steve Daines, R-Mont., from opposing the bill. Johnson said two weeks ago he would oppose the measure because he felt there was a big disparity that would lead to millions of businesses being taxed at higher rates than other firms because of the way they are structured.

Daines spoke with Trump about his concerns Sunday night, and Daines has also spoken with Vice President Pence and Treasury Secretary Steven Mnuchin in the past week. An aide to Daines said he was optimistic about the way discussions were progressing but his spokeswoman, Marcie Kenzel, said the lawmaker is currently a “no” vote on the bill.

Sen. Susan Collins, R-Maine, meanwhile, has said she wants the Senate bill changed in a way that allows Americans to deduct their local property taxes from their federal taxable income. A tax-cut bill that passed the House of Representatives allowed Americans to deduct up to $10,000 in property taxes from their income, but the Senate bill currently doesn’t include such a provision.

Sen. Jeff Flake, R-Ariz., has also expressed concerns about how the tax bill would add to the debt, and he has said he is pushing for changes that would account for the cost of extending tax breaks that are currently only scheduled to last for several years.

Sen. Lisa Murkowski, R-Alaska, and Sen. John McCain, R-Ariz., have issued generally supportive statements of the tax plan but have also said they are undecided. Both Murkowski and McCain have shown a willingness in the past to buck the Trump administration in pivotal situations, opposing Trump’s earlier effort to repeal the Affordable Care Act.

Failing to appease the lawmakers demanding changes could cost Trump his first major legislative victory, but adding too many changes could complicate the bill and also hurt its chances. If the Senate passes its tax measure by the end of this week, GOP leaders would need to reconcile differences between their tax bill and the one that previously passed the House of Representatives.

That could either happen in a formal “conference” process or the House could simply vote to approve the Senate bill in its entirety, sending the bill to Trump so that he can sign it into law.

Senate Finance Committee Chairman Orrin Hatch, R-Utah, met with Trump and several other GOP senators on Monday as they prepare for the final push and was asked if he thought a final bill would be signed into law by Christmas.

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